The Replacement
by Foundtoarvvie
Summary: An ordinary summer day was dramatically changed for the Flynn-Fletcher brothers as they are greeted by Ferb's biological brother. But something doesn't feel right with that kid and Phineas plans on figuring it out.
1. Chapter 1

**Replacement**

**Chapter 1**

It was a bright and shiny morning as the sun's light engulfed the whole city of Danville. It was summer so the natural cool breeze was a perfect combination to the warm weather. One specific household in the suburbs was the home of the Flynn-Fletcher family. Everyone was sound asleep except for one ten year old boy. Phineas Flynn was a famous young inventor along with his stepbrother Ferb Fletcher. Out of the duo, Phineas was the optimistic, friendly, and overall talkative one while Ferb was the calm collected one. Today was like any other ordinary summer day, but Phineas did not get a wink of sleep the previous night. Why you may ask? Well the triangle-headed boy was busy working on something spectacular for him, his brother and friends for the next day. And people who knew Phineas was that he wanted to make the best summer ever and nothing was going to stop him: not the laws of physics, the refusal of adults, and most importantly not the need to sleep. He has never done it before so he thought it would be a good idea to get some extra project ideas in. So it was now eight o' clock in the morning and he was already outside in the backyard, setting up his materials and tools. Phineas turned his head when he saw his stepbrother walking out of the house and gave him a big grn. "Morning Ferb!"

Though Ferb did not reply, he was the mut of the duo and only spoke his one line a day. But his brother knew how to read his expressions. 'Why are you up so early?'

"I've been working all night to plan for a project today so I'm just preparing the tools," Phineas answered his brother's unasked question. All of a sudden, their gate door opened revealing their neighbor friend.

"Hey Phineas! Whatcha doin'?"

"Hey Isabella! We're gonna be working a high powered motor-" Phineas was cut off by his mother.

"Sorry boys but Ferb has to come with me to the airport. I think it will be a surprise to all of us," Linda said as she came out of the house. The duo looked at eachother with confused looks but shrugged it off as Linda went away with Ferb to her car.

"Deja vu. I guess it's just us again Isabella," Phineas told his best friend. She replied with a loud squeal but then slapped herself across her face.

"Cool. Yeah, totally," Isabella said calmly.

"Oh, that's too bad Ferb had to go. I was looking forward to today's project but *yawn* I guess we have to find something to do," Phineas told her tirdley. Isabella noticed this and asked with concern laced in her voice. "Are you alright Phineas? You sound tired."

"Oh, well I stayed up all night working on the plans for today's project. Just a little tired is all."

"Well do you want me to bring the Fireside Girls Handbook? It might inspire you for another project."

"Sure, thanks Isabella!" As the fireside girl left, Phineas withdrew himself to his tree and propped himself against the trunk. He decided to take a little nap while Isabella returned as his fatigue was getting the best of him as his eyes slowly closed. A few minutes later, Isabella walked back into the backyard.

"I'm back, Phi-" She stopped to notice the snozzing inventor against the tree. 'He looks so cute when he sleeps' Isabella thought dreamily. Everyone knew she had a massive crush on the younger inventor, except Phineas himself. Slowly she made her way to the tree and sat down near him, and because of the movement, Phineas' head dropped to lean on the girl's arm. Isabella tried so hard not to squeal in excitement as to not to wake him up, but her blush was hard to hide. After a few solid minutes, the gate opened once more revealing the neighborhood bully and nerd, Buford and Baljeet.

"Hey Isabella," they greeted.

"Hey guys," she greeted back quietly.

"Where's Dinnerbell and Ferb?" Buford asked, not seeing the young inventor.

"Ferb's at the airport with Mrs. Flynn- Fletcher and as for Phineas, he's right here." Isabella pointed to the sleeping boy. The boys nodded. They all soon heard a car pull up into the driveway and next thing they knew was Ferb and another boy, who looked like Ferb but had blond shaggy hair instead of green and was way taller , walked in. The silent brother saw his normally ecstatic brother sleeping so he decided to wake him up with his homemade alarm clock. Curious, the others watched while Ferb turned it on. What surprised them was that not only did Phineas wake up with a jolt and his hands swung to his ears, but they couldn't hear anything.

"Ferb! Turn that thing OFF!" Phineas yelled in annoyance.

Ferb did so.

"I told you not to use that thing again," Phineas glared at his stepbrother for his only response was a shrug.

"Uh, Phineas… we didn't hear anything," Baljeet spoke.

"It's supposed to be an alarm clock that only I can hear with certain change in settings. The sound is super annoying and I told Ferb never to use it again," the young inventor told his group of friends. That's when he noticed the kid that came in with Ferb. "Hey, who are you?"

"Name's Calvin, I'm Ferb's biological brother."

**To be continued...**


	2. Chapter 2

**Chapter 2**

"Wow! Really? Nice to meet you! I'm Phineas," the redhead said enthusiastically. He held out his hand with which Calvin shook but something was off. Phineas noticed the other boy's smile and his general vibe sent chills down Phineas' spine. The once happy grin was replaced with a small frown, but noone noticed this except the Fireside girl. Once they retreated from their shake, Phineas turned to his step brother. "You never told me about your biological brother."

"I didn't know I had one," Ferb confessed. Phineas was usually the optimistic one but something about Calvin set off many red flags in his mind. He was growing suspicious but tried not to show it. He knew his friends would think he's crazy if he thought Calvin was off and his brother would definitely not forgive him. So he decided to play it normally, but he would be keeping an eye on him.

"Our biological mother divorced our father but the deal was while I took my mom, Ferb went with dad," Calvin explained. The others nodded in agreement. "Back when we were together though, we were inseparable as we built many things like toys and what not. On the way here, he told me that you guys do the same."

Phineas and Ferb smiled proudly at the thought.

"Hey, can I use your restroom?" Calvin asked.

"Sure, I'll show you the way," Phineas offered. He was thinking that he knew he was overreacting about Ferb's biological brother. It was absurd. He was getting used to the new kid even though he just met him. "Here we go it's right in he-" Phineas was cut off by an arm pinning him against the wall.

Calvin's arm.

"Calvin?! What are you doing?!" Phineas asked frantically.

"I saw you eyeing me. You think I'm suspicious looking and I don't like it. I will tell you one thing and one thing only… Ferb's my brother. You never belonged with him and I will do anything in my power until he forgets you." He roughly let go of Phineas and left to go back into the backyard. Phineas just stood there, shocked. After a few moments to gather himself, the young inventor headed back into the yard slowly. 'I won't let this guy take Ferb away from me' Phineas repeatedly thought.

"There you are Dinnerbell. We thought you got lost in your own house," Buford jokes. The redhead paid no attention to the comment and glared daggers into Calvin, who smiled sinisterly. No one noticed this though.

"Ferb, I know what we're going to do today," Phineas said with his catchphrase but his eyes never left the 'biological brother'. The whole team did notice this but shrugged it off as nothing. They all waited expectantly for Phineas to tell them what they were going to do and how but CCalvin was the first to speak. "Hey Phineas? Why don't I help you with the plans and building. It would be just like old times with me and Ferb."

"... Fine," Phineas muttered and motioned his head for Calvin to follow him. They both returned from the garage with a blueprint and a tool box. The gang looked at what they were doing and started building (the supplies were already called by Isabella). Throughout the session, everyone was having a good time.

Except Phineas.

He was constantly being sabotaged with his work by Calvin and everyone was getting frustrated with his 'mistakes'. Though they all thought the redhead was having an off day, he didn't sleep at all at night after all. At one point even Calvin was getting annoyed because his plan wasn't working, so he tried a different approach. As Phineas was getting some bolts from the tool box, Calvin snuck behind Phineas and SLAMMED the tool box lid on the inventor's exposed hands which he immediately cried out in pain. Everyone stopped what they were doing and hurriedly went to check Phineas, while Calvin tried to act innocent as possible but Phineas knew it was a lie.

"Phineas! What happened?" Isabella urgently asked as she helped open the tool box revealing Phineas' hands with a blood marked line across them. Everyone gasped at the injury and soon their expressions were mixed with confusion, worry, and… satisfaction.

Calvin was satisfied.

"Nothing," the redhead muttered as he clenched his fists until they turned white as he glared daggers to Calvin. Ferb noticed this and decided to investigate later.

"I see Phineas knows how to take his pains like a man!" Calvin declared happily as he patted the young inventor on his shoulder. Though his only response was a low growl, though so quiet no one heard this.

"Let's just get back to work," Phineas mumbled as he harshly retracted from Calvin's hand. Only then everyone noticed the change in their optimistic friend. Since Calvin arrived, Phineas had been more harsh, annoyed and has given an attitude they never experienced before. But what they noticed more is that only Phineas did this to Calvin.

It was sundown and they had already finished riding the ride. Everyone enjoyed themselves, especially Phineas as he got to relieve some stress since the arrival of Calvin. As no one noticed, like usual, the ride magically disappeared when a green beam hit it. Everyone bid their goodbyes and all went home, except for the three boys. They all entered the house and was greeted by Mr. and Mrs. Flynn-Fletcher.

"Hello boys. Just to let you know, Calvin, you're going to be sleeping in Phineas and Ferb's room in a sleeping bag. Now go wash your hands, it's time for dinner then bed time," Linda said. The boys nodded in response and did what they were told. As they had dinner, it was noisy. Calvin, Linda, and Lawrence talked to each other while Phineas and Ferb ate their food in silence. Ferb noticed this and tapped his step brother's shoulder. Once the redhead looked up, Ferb gave a look that said 'Are you okay?', and knowing what he wanted to say, Phineas answered his unasked question as he forced a smile. "Yeah, I'm good bro."

Ferb did not believe this though and decided to drop it until after dinner. As bedtime arrived, Calvin went to brush his teeth first while the other two put on their pajamas, that's when Ferb decided to make his move. "Are you okay?"

"Yeah I'm fine man!" Phineas lied as he scratched his ear. But his step brother noticed this and called it out.

"You're lying. You always scratch your ear when you're lying. And now I know that something is wrong because you never lie."

" Since when do you ever talk this much?"

"Since when I started worrying for my brother."

"It's nothing. You wouldn't believe me even if I told you," Phineas sighed.

"Phineas. You're my brother and don't ever think that I won't believe you, got it?" Ferb spoke seriously.

Phineas nodded.

"Now tell me what's wrong."

"Calvin was the one who slammed the lid of the tool box on my hands," Phineas told him as he held up his blood dried hands. Ferb was shocked, to say the least. If it was true, why would Calvin do that? "What?"

"He was the one that did this. He was the one that made me mess up earlier. He was the one-" Phineas cut himself off. He started biting his lip as he hesitated. He didn't know if he should tell Ferb about what Calvin said about him being taken away.

"'He was the one' what?" Ferb inisted.

"He was the one who pinned me against the wall," Phineas decided to say. It was technically the truth as it did happen, it just wasn't the whole truth. But he was startled when Ferb did something he didn't expect.

Ferb started laughing.

"What's so funny?" Phineas asked, annoyed that his brother wasn't taking the matter seriously. Once he calmed down, Ferb spoke.

"Well don't you think it's a little stretch that my biological brother is doing those things to you?"

"It's true! You said you'd believe me!"

"What I do believe is that you need a little sleep. Pulling that all nighter must have messed you up, but don't worry. If Calvin is a problem, I'll sort him out. Promise?" Ferb said as he held out his hand. Although he was a little upset Ferb didn't believe him, Phineas shook his hand.

"I'm holding you on that."

"You're holding on what?" Calvin came into the room. Phineas and Ferb exchanged glances but they just shook their heads as they didn't want to talk about it.

"Okay whatever. Let's head off to bed."

**To be continued...**


	3. Chapter 3

**Chapter 3**

It was early the next morning. Phineas jumped out of his bed and dressed into some dark blue jeans and his orange and white striped shirt with a dark green jacket. He quickly put on his shoes and headed to the bathroom. The redhead opened the sink's drawer and took out Ferb's comb. He pulled out one of Ferb's green hair from it and put on some toilet paper then shoved it in his pocket. He put the brush back, closed the drawer and headed out of the bathroom and down into the kitchen. He checked that the oven clock read '6:30 am'. Once he listened for if anyone woke up when he was doing his stuff. Luckly, Phineas was in the clear and he headed outside and made his way to the city area of Danville. He knew he was heading to his friend's father, more specifically Vannessa Doofensmirtz. Remembering that she didn't know he was on his way, he pulled out his cell phone and dialed her number. It rang for a few seconds until a girl's voice came up. "Hello?"

"Hey Vannessa! Sorry for waking you," Phineas said apologetically.

"It's fine Phineas. I was just at my friend's party and I'm headed home now. So what's up?"

"Oh cool! I wish I was old enough to go to those kinds of parties! Anyways, I was wondering if actually you dad can help me with something."

"You will get your chance. Maybe I can even invite you to the next one!"

"That'd be awesome."

"But for what you're asking, you could come over to my place so you can talk to him. I can pick you up along the way since I'm heading over there. Where are you now?"

" I'm near my house. Do you remember the address?"

"Yup! Alright I'll be there in five." Vannessa said then she hung up. Phineas sighed and waited for his ride. Not too long later, a black sedan pulled up to where Phineas was. The window rolled down to reveal a goth at the driver's seat. "Hop in."

"Thanks Vanessa," Phineas thanked as he climbed into the passenger's seat. Once the car started the move, Vanessa started talking. "So why do you need my dad's help anyway? He's not good at anything, though he thinks he's good at evil."

"Well I actually need help with this hair sample," Phineas told the goth as he took out the toilet paper.

"Who's it from?"

"Ferb. I need to check his DNA in this strand of hair to check his genetics and heritage."

"And why do you need to do that?"

"Well… it's his biological brother. At least that's what he says but I think he's not what he says he is. I have trusted people before but this guy…"

"Is like a serial killer under disguise?"

"Yeah. I'm very suspicious of him. He even told me that he'll do anything in his power to get Ferb. He even smashed my hands in a tool box! Whoever this guy is, he's nuts."

"... But what if he is Ferb's biological brother? Then what?"

"I know that there is no chance that it's true. They might look the same but his story is far from the truth."

Silence.

"Well from what it sounds like, I agree that Ferb's 'biological brother' is a total lunatic. So if I were you, I'd go with my instincts. He's your family and family protects each other even if they hurt them in the process," Vanessa told him sincerely.

Phineas nodded.

The rest of the car ride was silent. The sun had already risen as they reached Vanessa's second home. "Doofenshmirtz Evil Incorporated!" A jingle sang as they made their way inside the building and into the apartment. The goth turned to the inventor as they entered the large space. "Wait here. I'll go get my dad, he should be already building his inator right about now." Phineas nodded as she went into a room. The redhead just started walking around the large room, curiosity taking over him. He stopped when he heard two pairs of footsteps behind him. He turned around to face the goth and the evil scientist. "Hey Dr.D."

"Hey kid. Vanessa filled me in on what you told her and I agreed to help. Can you hand me the hair strand?" Heinz Doofenshmirtz asked which the boy complied.

"Do you think you can give me the heritage and genes from the DNA?" Phineas asked as they all walked to a table where a couple of tools were and a machine that looked like an old fashioned computer.

"It's a piece of cake. Especially with my new creation, THE HERITAGE-INATOR!" Heinz praised. He received confused and expectant looks from both kids so he decided to explain. "Well you see, I was raised by wild ocelots but I wanted to know where my real parents were so I could find them, but unfortunately the technology wasn't as advanced back then as it is now, heh. But I decided that I would make a machine that would be easier to find people and/or their genetics quicker."

"Woah! So how does it work?" Phineas asked in awe.

"Well this is just like a computer, but instead of putting in like a CD, you insert the sample into the slot and the inator will gather all the information it could find on that person. Now, let's test it out!" Dr. D placed the green hair strand on a tray which inserted itself inside the computer. Once finished, a little 'ding' went off and showed a bunch of information of it's screen. It read:

_Ferdinand William Fletcher:_

_Sex: Male_

_Age: 10_

_Blood type A_

_Father: Lawrence Fletcher_

_Mother: Unknown_

_Siblings: None_

The list read on and on but Phineas only stopped until he read the last two words- 'Siblings: None'. The young boy was feeling mixed emotions: righteousness, confusion, and worry.

There was a stranger posing as Ferb's biological brother.

That stranger was with Ferb right now.

"Dr. D! Can you print that out for me?" Phineas asked frantically. The scientist was a bit startled by the change in attitude but shrugged and did what he was asked to do. The redhead then turned to the teenager who had a shocked face.

"I'll give you a ride, don't worry," Vanessa answered his unasked question.

"Here you go-" Heinz was cut off by Phineas snatching the papers from his hand.

"Thanks Dr.D!" Phineas yelled as he and Vanessa ran out the door, leaving Doofensmirtz dumbfounded. The two kids hopped into the car and sped off heading to the Flynn-Fletcher home. Getting there in less than two minutes, Phineas hopped out of the car with the papers in hands.

"Hey Phineas!" Vanessa called. He turned his head towards the car. "Be safe, you don't know what he's capable of."

Phineas nodded.

The boy ran into the driveway, noticing a machine revealing itself over the fence, and opened the gate.

**To be continued...**


	4. Chapter 4

**Chapter 4**

The whole gang was there. Ferb, Isabella, Buford, and Baljeet. Even Candace was there. But there was one person Phineas did not want to see.

Calvin.

All eyes were on him but Isabella was the first to speak.

"Thank goodness Phineas, your back! Where've you been?!" Isabella asked anxiously.

"Yeah! We've even made a machine to find you," Baljeet added.

"If that thing didn't work, I thought you would've ran away or something," Buford chuckled. But Phineas ignored all of them and locked eyes with Calvin's. It was like a lion challenging the other lion to win back the pride. Phineas' eyes were fierce and blazing with rage as all of a sudden he charged at Calvin and pinned him against the tree he was standing near.

Everyone was shocked.

"Who are you?" Phineas asked dangerously.

"I'm Calv-"

"WHO ARE YOU?!" Phineas shouted, now everyone was scared. The never seen the optimistic boy this angry. Ferb pulled at Phineas' arms once his shock was gone. "What are you doing?!"

"Here," Phineas shoved the printed papers to Ferb," I looked up your heritage using the DNA from your hair strand. Read it." Everyone read the papers under Ferb's grasp and gasped when they reached the part about his nonexistent siblings. Phineas turned back to 'Calvin'.

"I knew I shouldn't trust you," the young inventor hissed. He grabbed the scared boy's shirt and made him look at him in the eyes. "Who are you?!"

"I-I'm C-Calvin Fletcher," the boy stuttered. This only made Phineas more furious as he lifted his hand and punched the imposter in the face.

Once again, the group was shocked.

Phineas continued with the punches as he spoke.

"Who. Are. YOU!" With one final blow, Phineas tossed the bloody boy to the ground.

"M-my name is N-Nick Freeman," Nick spoke weakly.

"Why? How?" The redhead interrogated.

"I used a machine that shot Ferb and your mother and father to believe that I was a part of their life."

"You still didn't answer my question: Why?" Phineas impatiently asked. The others wanted to know the answer as well. Even Perry, their pet platypus, arrived to know what was up with the imposter.

"I wanted to re-replace you. You don't deserve a brother. I had a brother too but he didn't deserve a person with a gift like mine," Nick smiled wickedly. Phineas' eye twitched with fury but decided to hold it off until the right time. "What did you do to your previous brother?"

"The same thing I'm about to do right now with you… murder." Nick jumped up and punched Phineas knocking him back a bit but regained his composure.

"Alright. We'll just see who falls first," Phineas said as he charged at his opponent who did the same. If there was one word the described the fight:

It was violent.

The fighting boys were punching, kicking, and wrestling. They were bloody from each others cuts and injuries but they were not letting up. Phineas heard cheers and turned his head in curiosity.

It was his friends- no.

It was his family.

His focus on the cheers became an advantage to Nick as he pinned his opponent down and started choking him. Phineas tried to gasp for hair but his throat was collapsing under the pressure.

"You shouldn't have messed with me. Your weakness is all you are and like I said before, Ferb doesn't deserve a brother like you UGH!" Nick fell to the ground as he clutched his head in pain. Phineas gasped for air and looked up to see a hand waiting in front of him.

Ferb's hand.

With his help, Phineas came up from the ground and the both kneeled to Nick's level, both with pissed off expressions.

"No Nick. I'm not weak. You are. And you know what else? You're pathetic. You couldn't even kill me!" Phineas chuckled but his face returned with an angry frown.

"You used me in the wrong way Nick. Now you'll have to pay by none other than the Flynn-Fletcher brothers," Ferb smirked evilly. Phineas did the same. In one swift motion, the step brothers grabbed a hold on Nick's shirt and together, they knocked him out cold.

"I'm sorry for not believing you," Ferb apologized. His response was a tight hug from his slightly younger brother, which he returned. Once they pulled apart Phineas spoke. "At least you had my back until the end. That's what really mattered."

Ferb gave a thumbs up.

They both stood up from their position and turned around to see the shocked faces of the group, even Perry had a shocked expression.

"Sorry guys. Just had to take care of the… threat, I suppose," Phineas awkwardly told them.

"Woah Dinner Bell, when did you ever learn how to do that?" Buford asked in awe.

"I've always known, same as Ferb. But we decide not to use it until the time is right," Phineas explained.

"Ohhh! Nick is sooo busted! I'm telling mom! That boy is going down! DOWN, DOWN, DOWN! D-O-W-N, DOWN!" Candace yelled hysterically as she ran into the house.

"Oh Phineas! You were so brave!" Isabella yelled as she ran to Phineas and gave him a big hug. Only the inventor cried in pain from all the bruises and cuts. Isabella retracted and started apologizing immediately. "I'm sorry! I'm sorry!"

"It's alright Izzy. Just hurt from all these cuts and what not," Phineas chuckled. Isabella blushed a little because of the new nickname. "'Izzy'?"

"Oh, uh… I just thought I gave you a nickname. I could just call you regularly if you don't like it," Phineas said, with a hint of disappointment in his voice. Isabella noticed this and hugged him again only gentler that time. "No. I love it."

Phineas smiled as he hugged back.

"Phineas. How did you know Nick was not who he really was?" Baljeet asked as the two kids broke their hug.

"I knew when I first shook his hand," Phineas sighed. "When I took him into the restroom yesterday, he threatened me that he would do everything in his power to separate Ferb and I. He sabotaged my work and crushed my hands with the tool box. That's when I knew that I had to do something so I asked for some help this morning from Vanessa. Her dad is a … type of scientist that helped me figure out Ferb's heritage."

"Thank bro," Ferb said.

"No problem. Anything for my brother," Phineas said with the first grin he had in awhile. His smile was contagious as it spread throughout his family. He heard groaning behind him but Phineas knew who it was. He rolled his eyes and turned around.

"What? Ready for round two?" Phineas spoke threateningly to Nick.

"Y-your crazy man," Nick said frightened as he vigorously shook his head.

"Oh I'm the psycho? Weren't you the one that tried to kill me a couple of minutes ago?" Phineas crossed his arms, starting to get annoyed.

"You should go to a mental asylum. That's where you belong, not here," Nick said, getting up. Phineas was too quick though and pinned him against the side of the house. Ferb tied the murder's hands behind his back. Once he was done, Phineas gave one hard slam into the wall then threw Nick in the middle of the circle of friends. Everyone had an angry expression on their faces, like they were ready to kill.

"If I'll belong in an asylum," Phineas started laughing maniacally. "Then I'm taking you with me."

The cops were called after that and took Nick away. Though he told his enemy one last thing before he was pushed into the car.

"I'll be back, Phineas, and when I do be ready. It won't be as easy as it was today."

" I wasn't even trying," Phineas taunted.

After the police left, everyone went home, Isabella hugged the redhead once more which caused him to slightly blush but luckily it went unnoticed. Once she left, the two step brothers were left in silence. After a while, Phineas spoke.

"... I'm sorry."

"What?"

"I'm sorry that you had to go through that. It should have ended differently than it did. I shouldn't have acted the way I did but- I was just so angry. So much rage filled my thoughts that it controlled my actions."

"It's okay. I forgive you. I would have done the same if it was the other way around."

They smiled at each other.

"Thanks bro."

**The end**


End file.
